Tom’s Restaurant is a real restaurant in New York City, on the corner of Broadway and West 112th Street. It supplied exterior shots for Monk's Cafe (informally known as "The Coffee Shop") in Seinfeld. The interior of Tom's differs from the interior of Monk's Cafe, for which Tom’s stood in. In the early episodes of Seinfeld on NBC, the entire “Tom’s Restaurant” sign was shown; in later episodes, it only showed “Restaurant”.
There is now a documentary narrating the story of diner: